Dads always told us that properly inflated, rotated, and balanced tires will do a lot for the way your car handles, including all but eliminating the need for pricey alignments. He never mentioned gas mileage, but he should have. Cartrackers.com lists properly inflated tires high on its list of ways to improve your car's fuel economy, and adds that keeping tabs on your tires helps them last longer. Since we haven't figured out yet exactly how to dispose of all those used-up tires, that's a motion I'll have to second.
Good thing Santa left all those tire pressure gagues in my Christmas stockings!